1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal device to be used for a liquid crystal display apparatus or a printer head. (Hereinafter, liquid crystal will be referred to as "LC"; ferroelectric liquid crystal will be referred to as "FLC"; and ferroelectric liquid crystal devices will be referred to as "FLCDs".)
2. Description of the Related Art
In recent years, LC display apparatuses of various display modes utilizing a LC material in the smectic phase have been vigorously studied in addition to LC display apparatuses utilizing a LC material in the nematic phase. Among display apparatuses incorporating LC display devices utilizing a LC material in the smectic phase, a display apparatus utilizing a surface-stabilized ferroelectric LC device (hereinafter referred to as SSFLCD) is considered particularly promising. Such a display apparatus is disclosed in literature by N. A. Clark, et al., Appl. Phys. Lett., 36, 899 (1980), for example.
An SSFLCD is known to have two orientation states: a uniform state, which exhibits some extinction positions; and a twisted state, which exhibits no extinction positions.
As shown in FIG. 25, a FLC layer 52 of an SSFLCD 50 has smectic layers 54, which are bent as shown in the figure. Such a structure is called a chevron layer structure. FLC molecules in the chevron layer structure take two orientation states, namely, C1 and C2 states, and one FLC layer may include regions of the C1 state and regions of the C2 state, as described in literature by J. Kanbe et al., Ferroelectrics, 114, 3 (1991). In the C1 state, the smectic layers 54 are bent or project in a direction (indicated by arrow L1) which is opposite to the direction (indicated by arrows R1 and R2) of a uniaxial orientation treatment that is performed for the alignment films of the device. In the C2 state, the smectic layers 54 are bent or project in a direction (indicated by arrow L2) which is identical with the direction (indicated by arrows R1 and R2) of the uniaxial orientation treatment for the alignment films. If regions of the C1 state and regions of the C2 state are both present in the same device, there may be a hairpin defect 56 or a lightning defect 58 in the interfaces of these regions. Reference numeral .differential.p represents a pretilt angle. The direction of pretilt depends on the direction of a uniaxial orientation treatment, e.g., a rubbing treatment.
Furthermore, FLC molecules of the chevron structure are known to take the four orientation states shown in FIG. 26, namely, a C1U (i.e., "C1 Uniform") state, a C1T ( i.e., "C1 Twisted") state, a C2U ( i.e., "C2 Uniform") state, and a C2T (i.e., "C2 Twisted") state.
Koden et al., who are the inventors of the present invention, report in M. Koden et al., Jpn. J. Appl. Phys., 30, L1823 (1991) that they obtained a FLC cell with parallel-rubbed high-pretilt alignment films in which three orientation states, i.e., the C1U state, the C1T state, and the C2 state were all present.
On the other hand, Tagawa et al. report in A. Tagawa et al., Japan Display '92, 519 (1992) that they obtained a FLC cell with parallel-rubbed alignment films having a pretilt angle of 5.degree. or less in which four orientation states, i.e., the C1U state, the C1T state, the C2U state, and the C2T state were all present.
In general, the driving characteristics of a FLCD depends on the orientation state of the FLC molecules. Therefore it is preferable for the entire LC layer to have a uniform orientation. Furthermore, if the C1 state and the C2 state are both present as in the case of conventional FLCDs, there may be hairpin defects or lightning defects occurring in the interfaces of the C1 and C2 regions, thereby leading to insufficient display or reduction in contrast. Therefore, it is preferable for the entire device to have a uniform orientation. The more preferable orientation state of the LC molecules is the C2U state, as opposed to the C1U and C1T states, in terms of such factors as the driving voltage, response speed, and extinction characteristic.
The present inventors have found that a FLCD of the C2T state can also attain excellent driving characteristics and contrast, as do FLCDs of the C2U state, by using a driving method such as that reported by Surguy et al. (P. W. H. Surguy et al., Ferroelectrics, 122, 63(1991)) called THE "JOERS/ALVEY" FERROELECTRIC MULTIPLEXING SCHEME.
However, methods for selectively obtaining the C2 state over the entire LC layer (or the entire device) have not been studied in detail.